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Title: Controls on dolomitisation of Upper Cretaceous strata of North Africa and Western Mediterranean
Author: Newport, Richard Joseph
ISNI:       0000 0004 5353 2107
Awarding Body: University of Manchester
Current Institution: University of Manchester
Date of Award: 2015
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Despite the economic importance of dolomitised Upper Cretaceous strata, little work has been conducted to further the understanding of the controls on dolomitisation during a green-house period with low marine Mg/Ca ratio. This study presents a multidisciplinary, multi-scale approach using field, petrographical and geochemical data to understand the mechanism of dolomitisation in North Africa and western Mediterranean in order to determine the controls on dolomitisation of Upper Cretaceous strata. Two field areas have been chosen that exhibit similar facies and are time equivalent, located along the Jeffara Escarpment in southern Tunisia and in the Iberian Range of central Spain. Both areas comprise facies deposited in mid-ramp, platform margin, lagoon, inter- and supra-tidal environments. Based on the distribution of facies, a sequence stratigraphic model was proposed for both areas which show similar changes in relative sea level. In the Iberian Basin there is strong evidence of a dramatic rearrangement in basin architecture resulting from tectonic activity during the Upper Cenomanian, whilst on the Jeffara Escarpment deposition took place on a passive margin. Both areas show evidence of warm, arid climates; even though the Iberian Range was in a more northerly palaeogeographic position, there was a rain shadow which affected facies distribution and dolomitisation. Planar dolomite fabrics, dull luminescence under cathodoluminescence, marine trace element concentrations and marine to slightly enriched oxygen isotope values suggest that dolomitisation occurred from mesosaline, marine fluids. Dolomitisation occurred from multiple fluxes of dolomitising fluids, controlled by high frequency changes in relative sea level. A cessation of dolomitisation only occurred during late transgressive systems tract of low order sea level cycles as a result of platform flooding. The Distribution of dolomitising fluids was partly controlled by pre-cursor limestone with low permeability horizons acting as aquicludes and permeable sandstones acting as aquifers. Changes in basin architecture caused reversals in flow of dolomitising fluids on the Iberian Range whilst increased subsidence caused cessation of dolomitisation due to rapid increase in relative sea level on the platform top. Dolomitisation occurred over a wider geographical area in the Jeffara Escarpment compared to the Iberian Range as a result of higher fluid salinity, larger brine pool and higher temperature of dolomitising fluids. The distribution of dolomitised strata of the circum-Tethys and across the Arabian Plate suggests that early reflux dolomitisation occurred within arid climate belts, and did not require hypersaline conditions, evidenced by evaporite distribution, as previously suggested. This study has important implications for the hydrocarbon industry by improving our ability to predict distribution, size and geometry of dolomitised strata essential for hydrocarbon exploration and field development. Furthermore this study has improved our understanding on the controls on dolomitisation during greenhouse periods with low Mg/Ca ratio of seawater and high frequency sea level changes.
Supervisor: Not available Sponsor: Hess ; North Africa Research Group
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Dolomitisation ; Upper Cretaceous ; Mesosaline ; Jeffara Escarpment ; Iberian Basin